"It all depends on what state you're in and whether the seller has a physical presence (property or employees) in that state," writes the foundation's Joseph Henchman.

If you live in New York and you buy from Amazon.com, the company will collect sales tax from you. New York is one of 16 states in which Amazon is required to collect the taxes.

New York's highest court ruled this spring that Amazon and Overstock.com Inc. could be compelled by the state to collect tax from online sales. The two companies appealed, but the Supreme Court declined today to hear that appeal.